The present invention relates to an electrophotographic process for selectively making either a single copy or a plurality of copies from an original document.
The basic electrophotographic process to which the present invention constitutes a substantial improvement generally comprises electrostatically charging a photoconductive drum, radiating a light image onto the drum to form an electrostatic image through localized photoconduction, developing the image through the application of a developing or toner substance to produce a toner image, transferring and fixing the toner image to a copy sheet to provide a permanent copy and cleaning residual toner from the drum.
It has become popular to utilize a magnetic brush for applying the toner substance to the drum. A magnetic brush comprises a rotating magnetic core to which is magnetically adhered the toner substance. The drum is rotated in brushing engagement with the magnetic brush so that the toner is attracted and adheres to the high charge or dark areas of the electrostatic image.
Whereas separate means have been proposed to clean the drum, or remove residual toner substance therefrom after the transfer operation, it is advantageous to utilize the same magnetic brush which is used in the developing step after electrostatically discharging and/or uniformly radiating the drum with light to dissipate the electrostatic image. Such an arrangement provides a compact, economical and low maintenance copying machine since a separate cleaning means is eliminated. In addition, the arrangement provides automatic recycling of the residual toner substance.
However, a problem has remained heretofore unsolved which has required a design decision to made as to whether to tolerate ineffective cleaning and the resultant double printing or whether to increase the complexity of the machine to provide satisfactory cleaning. More specifically, the consecutive steps of electrostatically discharging, uniformly radiating and cleaning the drum with the magnetic brush are insufficient to effectively clean the drum. This is because a bias voltage is applied to the magnetic brush which is suitable for the developing step. This bias voltage is on the order of the electrostatic potential in the light areas of the electrostatic image so as to prevent the white areas of the copies from appearing gray. This bias voltage is not high enough to allow the magnetic brush to be effective during the cleaning step. If the bias voltage is increased during the developing step, the electrostatic image on the drum will be seriously degraded resulting in unacceptable copying quality.
Adding additional components to increase the bias voltage during the cleaning step only requires a dual potential bias voltage source and automatically synchronized switching means which add considerably to the complexity and cost of the copying machine and offset the advantage of utilizing the magnetic brush both for developing and cleaning.
Furthermore, in high toner density areas of the drum, it is impossible to completely discharge the drum by means of the corona discharge unit which is widely used. This is because the residual toner accumulation is so thick in these areas that the corona produced ions cannot penetrate through the toner substance to the drum. Whereas the uniform light radiation aids the discharge process through partial penetration of the light through the toner substance resulting in photoconduction, substantially complete discharge cannot occur unless the light intensity is extremely great. Such a high light intensity causes fatigue of the photoconductive layer of the drum resulting in extremely degraded copy quality, especially in multiple copy operations.